In the process of fabricating semiconductor devices, gettering techniques are employed in order to avoid property degradation resulting from contamination of heavy metals, such as Fe (iron) and Ni (nickel). Gettering is a technique that introduces a heavy metal atom into a gettering site in a semiconductor substrate, and keeps low the heavy metal concentration in the vicinity of the semiconductor substrate surface serving as a device active region. A bulk micro defect (BMD) that is grown from oxygen precipitate nuclei contained in the semiconductor substrate is utilized as the gettering site. In a semiconductor device fabrication process (device process), the BMD is grown in a process (thermal process) in which the semiconductor substrate is heated.
In recent years, however, circuit pattern miniaturization of semiconductor devices has progressed, and with such a circuit pattern miniaturization, the thermal processes in the fabrication of the semiconductor devices have been performed at lower temperatures. The thermal process, for example, at or below 1000° C. is in some cases employed. In such a low temperature thermal process, the growth of precipitate nuclei cannot be expected, and the gettering capability of the semiconductor substrate is low.
In order to solve this problem, a semiconductor substrate doped with nitrogen or carbon is in some cases used. By doping the nitrogen or carbon to the semiconductor substrate, the precipitate nuclei are apt to grow even in the low temperature thermal process. Such a semiconductor substrate can be obtained by cutting from a silicon single crystal grown from a silicon melt to which the nitrogen or carbon is added.
When an epitaxial layer is formed on the surface of the semiconductor substrate, the process of forming the epitaxial layer is performed at high temperatures and therefore, if the nitrogen or carbon has not been doped, oxygen precipitate nuclei in the semiconductor substrate are lost, and BMDs are not formed in the device process. In contrast to this, when the nitrogen or carbon has been doped in the semiconductor substrate, the BMD is grown in the epitaxial layer formation process and the device process.
In this method, however, due to segregation during pulling of a silicon single crystal, the concentration of nitrogen or carbon varies greatly between the upper and lower portions of the silicon single crystal, and following this, the density of oxygen precipitates also greatly varies. Because of this, of one silicon single crystal rod, a portion where the BMD of appropriate density and size is obtained is extremely small in amount.
In the device process, a method of forming oxygen precipitates in a semiconductor substrate (instead of doping the nitrogen or carbon) in which a semiconductor substrate (wafer) of high oxygen concentration is prepared to pre-anneal the substrate before the formation of the epitaxial layer has been proposed as another method of stably growing oxygen precipitates (BMD) that could be a gettering site.
For example, Patent Literature 1, described below, discloses a method of producing an epitaxial wafer, in which a wafer is cut from a silicon single crystal having an oxygen concentration from 18×1017 to 21×1017 atoms/cm3, and this wafer is heat treated (pre-annealed) at a temperature of 750° C. to 850° C. for 20 minutes or more to 50 minutes or less, and the epitaxial growth for this wafer is provided. Oxygen precipitate nuclei formed in the wafer by this method are not lost during the formation of the epitaxial layer. In this wafer, for example, in a region having a thickness of about 10 μm immediately below the epitaxial layer, oxygen precipitate nuclei of high density are formed and grows in the device process.
However, the pre-annealing process is requisite for such a method and therefore, production costs are increased accordingly.
Incidentally, an epitaxial layer is used as a device active region where a diode, transistor and the like are to be formed and therefore, when a dislocation occurs in this region, in some cases, degradation of electrical characteristics of the device (for example, leakage failure) occurs. In this case, the yield of the device is reduced. When the oxygen precipitates of high density are present immediately below the epitaxial layer, as described in Patent Literature 1, described below, and if a dislocation occurs resulting from the oxygen precipitates, the dislocation easily reaches the epitaxial layer serving as a device active region, causing epitaxial defects, thereby degrading the electrical characteristics of the device. Further, when BMDs of large size grow, the strength of the wafer is degraded.
Patent Literatures 2 and 3 described below, as a method of avoiding the above described epitaxial defect problems, disclose techniques in which a wafer is solution treated at high temperatures to cause the oxygen precipitate nuclei to be lost, and thereby to restrain the formation of the BMD.
However, the methods of the Patent Literatures 2 and 3, described below, are such that even if the gettering capability is lost, the amount of the BMD is reduced, so that epitaxial defects will not be introduced. Therefore, in the device process in which heavy metal contamination could occur, wafers produced in these methods can not be used.